The present invention relates to improvements in web slitting mechanisms, for traveling paper slitters capable of cutting multiple or single layer webs having considerable bulk.
In the development of paper web slitters which are capable of cutting a web and traveling at high speed, commercial slitters frequently employed an annular sharp edged blade on top which coacts with the side edge of a drum or band below the web and the coaction of the sharp edge of the knife and the side of the band perform the shearing action. The edge of the drum may be ground to form a slight angle of several degrees. The theory of shearing web material is to create as high a unit load on the surface of the material as possible at the cut point in order to penetrate and sever the individual fibers of the paper web. Any action which occurs at the point of shearing that does not precisely cut the fibers, but instead results in a tearing of the fibers will result in a poorer, less precise edge and will result in the generation of dust which can become very undesirable at all speeds. It has been found that cutting high bulk webs such as board or multiple sheets will create a degeneration of the cut as the amount of bulk or thickness increases thus creating a poorer cut and increasing the dust. In a conventional slitter as above described, the top slitter blade will create a higher unit load on the top surface of the paper than the bottom drum which offers no sharp point to the paper. Thus, the top knife does more cutting work than the bottom drum edge and the more bulk that the top knife must penetrate, the more the quality of the cut deteriorates as the blade passes further into the web. In a multiple web situation such as in a sheeter or a high bulk operation such as that for cutting board, the same theory applies.
In accordance with the principles of the present invention, the cutting edge at the bottom of the sheet is formed substantially the same as for the top of the sheet, and this is done in a manner without sacrificing the advantages of the former structures which cut against a drum. In the present arrangement, both the top and bottom cutting knives do equal work and, therefore, the amount of bulk going through the knives can be substantially increased. Instead of using a bottom drum which acts as an anvil, a sharp blade is employed in such a manner so that both the top and the bottom of the sheet are subjected to a similar high unit load. It has been found that this arrangement can be made operative in a manner that achieves satisfactory high bulk cutting at high speeds and substantially reduces the amount of dust created by providing a support for the sheet immediately adjacent the lower cutting knife in the form of an annular support band which has a support surface for the sheet of substantially the same outer diameter as the lower knife. The supporting band and lower knife can be carried on the same drum to rotate at substantially the surface speed or at a faster speed than that of the traveling web and can be turned by the web or be driven by a separate drive. Further adjustability for obtaining control and selection of the widths of sheets to be formed from a traveling web can be obtained by providing multiple drum knives and drum supporting band surfaces and making the drum laterally adjustable relative to the travel of the web. If the drum is nondriven, the traveling web is allowed to wrap the drum for driving engagement between the web and drum surface.
In high speed paper web cutting, the problem of dust is a serious one and creates limiting parameters of operation and thickness of bulk being cut. As to the thickness in which multiple sheets can be cut, this is limited by the quality of cut produced in the intermediate sheets; and as the number of multiple sheets is increased, a point will be reached wherein the quality of cut through the innermost sheets becomes unsatisfactory for commercial grade sheets. Various factors, of course, influence the dust created such as the furnish used, i.e., the amount of distribution of fines, the compressibility factor of the sheets, the total thickness, the coating used, etc. The generation of dust also can make it necessary for frequent shut-downs for replacement or sharpening of the knives thereby reducing the overall output of a commercial machine and increasing the cost of operation. Slit quality and dust are primary factors, and these have been found to be affected by sheet flutter, incorrect penetration of the cutter, improper loading, incorrect shear angle, slitter vibration, machine vibration, excessive wear, as speeds increase as well as thickness and bulk of the web increase.
It is accordingly an object of the present invention to provide an improved slitter of the type above described which is capable of cutting traveling webs and providing an improved quality of the cut edge and reducing the amount of dust generated by the cut.
A further object of the invention is to provide an improved slitter which is capable of operating at higher speeds and with the generation of less dust and is capable of handling higher bulk with satisfactory slitting operation.
A still further object of the invention is to provide an improved slitter which corrects problems heretofore present in previous slitters.
Other objects, advantages and features will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: